The peroxidase enzyme system in human saliva catalyzes the peroxidation of the thiocyanate ion to generate products which inhibit the growth and metabolism of many species of oral bacteria. The major product of the system at neutral pH is the hypothiocyanite anion which is a normal component of human saliva. Our knowledge of the biochemical and microbiological mechanisms by which the peroxidase system functions has expanded significantly during the last 10 years. However, the following important questions remain unanswered: (1) What is the molecular structure of human salivary peroxidase and how does this structure influence its function? (2) Do peroxidases derived from leukocytes and commensal microorganisms make significant contributions to the peroxidase activity in mucosal secretions? (3) Does the salivary peroxidase system exert its antibacterial effects on different organisms by the same or by different mechanisms? (4) By what mechanisms may the salivary peroxidase system interact with other mucosal defense systems? We propose to seek answers to these questions with experiments utilizing purified human salivary peroxidase as a key component. Such studies will require considerable quantitites of the purified enzyme; therefore, a primary objective of the proposed study is the isolation and characterization of milligram quantities of human salivary peroxidase. This project is feasible because the enzyme has now been purified to homogeneity in our laboratory, polyclonal antisera against the enzyme have been raised in rabbits, and data on the enzymes' amino acid and carbohydrate composition and molecular weight have already been obtained.